Magnesium and its alloys offer the combination of low specific gravity and relatively high strength for automotive body panels and other useful components. But magnesium alloys are subject to oxidation and other corrosive reactions in the often humid, oxidizing outdoor environment to which such automobile components are exposed. Further, magnesium is electrochemically anodic with respect to many other materials that might be used to cover or isolate the magnesium or magnesium alloy from oxidation.
Ferrous metal and other metals used in vehicle components can often be protected from corrosive attack by coating systems that are anodic to the structural metal portion of the part. For example, anodic zinc coatings are applied (galvanizing) to iron and steel parts to provide protection against corrosion of the base cathodic part for substantial periods of time. In this layered combination of zinc surface coating and iron substrate, the zinc is consumed first by water-promoted or air-promoted oxidation at the exposed surface of the part. Zinc is oxidized to a salt or other non-useful material and, thus, consumed sacrificially for protection of the underlying, stronger functioning material of the vehicle part. But no such anodic coating material has been discovered for magnesium and magnesium alloy parts and components.
The surfaces of magnesium alloy components can be painted or otherwise provided with barrier-type organic-based coatings or other barrier coatings, for protection that lasts as long as the coextensive coverage of the coating is maintained. But when some portion of the barrier coating is damaged and that portion of the underlying part is exposed, the exposed magnesium is anodic with respect to its surroundings and undergoes severe local corrosion which may be more damaging to the component than if it had not been coated in the first instance. There remains a need for more protective coatings for magnesium and magnesium alloy parts, coatings that are anodic in a water-containing environment to the underlying magnesium substrate.